@FrancisPUnfortunately all they have to do for this scam, is get a list of dialling codes, which are publically available and enter every combination into a spreadsheet and upload into autodialler software, use a cloud based phone system and away they go...
It can be built and running in about 12hrs, and deleted in 10minutes, with no trace...
Its not an easy one to fix im afraid, as they cant block all calls from these locations , so patience with o2 (and the other networks) and it will get sorted.
This is not customer services and we dont have access to your account
I do not work for o2 or any VMo2 /Telefonica/Liberty Global Company